Looking back on why we look forward: A special research series by Adam Glover, Part 5

The snake detection hypothesis draws support from neuroscientific studies which suggest that primate-specific regions of the visual system involved in the fast processing of threatening images (called the pulvinar visual system) evolved in part to assist primates in detecting snakes (Van Le etal. 2013). One such study utilized two macaque monkeys (Macacafuscata), neither of which had encountered snakes before the experiment. The monkeys were then set in front of a computer display and shown a set of color photos that included images of snakes, monkey faces (angry and neutral), monkey hands, and simple geometric shapes like squares and circles. The monkey’s eye position was tracked by an eye monitoring camera, and activity of pulvinar neurons was recorded 100 milliseconds (ms) before and 500 ms after presentation of the stimulus.

Logic would dictate that if the pulvinar visual system evolved to assist primates in the detection of snakes specifically, then the pulvinar neurons of primates would show larger, faster responses at the sight of snakes than at the sight of other threatening images. Additionally, if the pulvinar neurons of primates were to show larger, faster responses at the sight of snakes than at the sight of other threatening images, then it could follow that developing more acute vision  in the lower visual field through orbital convergence (so as to better spot snakes) would be selected for as well. Of the pulvinar neurons being observed, 14.1 % responded to at least one of the images, and all of those that responded were then tested again with all of the images. Of those that were then tested again, 40.6% showed significantly greater responses when shown a snake than when shown other stimuli (Figure 3). Thus, the results of this study provide neuroscientific support for hypothesis that the development of more acute vision in the lower visual field through orbital convergence was an adaptation to predation by snakes.

Fig 3. Mean response magnitude of pulvinar neurons to each image in the stimulus set. Asterisks indicate significant difference between image categories. (*P<0.05, **P< 0.01, ***P<0.001, Bonferroni test after one-way ANOVA) (Van Le etal. 2013).